Buttermilk

Hi Jen - if your dressing turned out to be less rich, thinnner consistency than you remember in the restaurant, the lowfat buttermilk might be the reason. But, I've ONLY seen fat-free and low-fat buttermilk in the supermarkets here - not full fat.
I have a couple of suggestions, though, to "enrich" your dressing if you can't find full fat buttermilk - replace a bit of the buttermilk in the recipe with a little mayonnaise. OR, buy some of the powdered buttermilk that is available in the baking section of the supermarket and add some to the low-fat buttermilk - that should make it taste richer.

Hope this helps out!